Electronics Engineering And Circuit Design

Common Base CB Configuration

Common Base Configuration

Look at the figure, the input is applied to the emitter terminal and output is obtained from the collector. Whereas base is a common terminal to both the input and output signal. The common base configuration has low input impedance and high output impedance. Unity current gain and high voltage gain.

Biasing Of CB Configuration:

Figure 1 (a) shows the CB configuration and figure 1 (b) shows a biased NPN transistor. Now let me explain how to bias a CB transistor configuration.

To analyse input and output characteristics of CB configuration, the transistor should be in active mode
For active mode, base-emitter junction is forward biased and base-collector junction is reverse biased
To forward bias base-emitter junction, connect the positive terminal of supply to base and negative terminal to the emitter.
To reverse bias the collector-base junction, connect the positive terminal of supply to the collector and the negative terminal to the base

Common Base Configuration Input and output characteristics curves, biasing

Input Characteristics Of CB Configuration:

The graph plotted between input current that is emitter current IE and input voltage (emitter to base voltage) VEB at constant output voltage (collector to base voltage) VCB. IE is plotted along the y-axis and VEB along the x-axis.

The input characteristics of CB configuration is exactly similar to CE input characteristics. In CE, the input characteristics curve is plotted between IB and VBE. Since the base emitter junction is forward biased, we expect a similar graph to a forward-biased diode.

Understanding the effect of VCB on input characteristics curve:

IE increases with increasing VEB. This is the property of a forward-biased diode. Nothing is new. Now understand the effect of VCB on IE.

As you know reverse bias enhances minority carriers. VCB is a reverse biasing voltage. It reverse biases the collector-base junction. The higher value of VCB increases the width of the depletion region at the base-collector junction. As a result, the effective width of the base decreases. Due to a decrease in effective base width, the concentration gradient of holes in the base increases. In other words, there are more charge particles (holes) per unit area. The increased concentration of holes in the base region causes the diffusion of electrons from the emitter. This increases emitter current IE.

And hence, an increase in VCB will increase in IE.

Output Characteristics Of CB Configuration:

It is the graph plotted between output current (collector current) IC and output voltage (collector to base voltage) VCB at constant input current IE. IC is plotted along the y-axis and VCB along the x-axis.

By observations, the output characteristics of CB is quite similar to CE and CC configuration. The difference is, the saturation region is on the left side of the y-axis (VCB is negative).

In figure 1(d) each curve starts at IC = 0. IC increases as an increase in VCB. Forward biasing collector-base junction (VCB) causes IC to increase exponentially.

Current Amplification Of Common Base Configuration:

The current amplification of common Base configuration is less than unity. As you know the current amplification factor is defined by the ratio of output current to the input current. Here input current is IE and outputs current is IC. Hence the current amplification factor is

α = IC / IE

Since IC is approximately equal to IE , but always less than IE. Hence α is less than unity.

Input Resistance Of Common Base Configuration:

It has low input resistance. In this configuration, the input resistance is the ratio of base to collector voltage VBE to the base current IB. At constant output voltage VCB

Ri = VEB / IB

Since IB is very small, VBE is also small (VBE = 0.7V). Ri is much smaller than CE and CC configuration.

Output Resistance Of Common Base Configuration:

In CB configuration, output resistance is the ratio of output voltage VCB to the output current IC. At constant input current IE.

RO = VCB / IC

Voltage Gain Of CB Configuration:

As I discussed earlier, voltage gain is the ratio of output voltage to the input voltage. The output resistance of BJT in the common base configuration is very high. The load resistor (has a small resistance) is in parallel output resistance of BJT. All current will flow through RL.

AV = vcb / veb

AV = icRL / ieRi

AV = αieRL / ieRi

AV = αRL / Ri